The long term goal of this project is to determine the mechanism of recombination of virus DNA with the host chromosome (site-specific recombination). Recombination between bacteriophage Lambda and its host Escherichia coli involves specialized regions in each DNA called attachment sites and three proteins: the phage int and xis proteins and the host IHF protein. We have determined the nucleotide sequence of the gene (hip) that encodes one of the subunits of IHF (IHFBeta). One of the polypeptides predicted from the nucleotide sequence has an N-terminal sequence that is found in IHF and, like IHFBeta, is basic with a molecular weight between 10,000 and 11,000 daltons. These nucleotides, therefore, almost certainly encode IHFBeta. We have also completed our characterization of an int mutation than retains partial activity; the results suggest that int protein can discriminate between different attachment site pairs after synapsis has occurred. We are continuing our studies of a second int mutation that increases int activity and also confers partial independence of IHF and xis proteins. We have located this mutation within a small segment of the int gene by fine structure mapping.